This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-137017, filed May 10, 2000; and No. 2001-014051, filed Jan. 23, 2001, the entire contents of both of which are incorporated herein by reference.
The present invention relates to a method of monitoring the neutron sensitivity of a neutron detector.
A general neutron detector, e.g., a nuclear fission ionization box, for monitoring the neutron flux in an atomic power station includes an anode 11 and a cathode 12 housed in a chamber 10, as schematically shown in FIG. 1. The surface of at least one of the anode and the cathode is coated with a nuclear fission substance 13, and an ionizing gas is loaded in the space between the anode 11 and the cathode 12 within the chamber 10. A DC power source 15 is connected to the anode and the cathode with an ammeter 14 arranged between the anode 11 and the DC power source 15.
If the nuclear fission ionization box of the construction described above is irradiated with neutrons Y, the ionizing gas is ionized by the nuclear fission pieces. The ions and electrons formed by the ionization are collected to the anode and the cathode, with the result that an electric current flows between the anode and the cathode. The current flowing between the anode and the cathode is measured by the ammeter 14, and the neutron flux is monitored by utilizing the fact that the magnitude of the current is proportional to the neutron flux.
In the case of the nuclear fission ionization box, the neutron flux is monitored by utilizing the fact that the current flowing between the anode and the cathode is proportional to the neutron flux irradiating the nuclear fission ionization box. Therefore, in the nuclear fission ionization box, the nuclear sensitivity is monitored in advance, and the neutron flux is monitored on the basis of the neutron sensitivity monitored in advance.
In the conventional method of monitoring the neutron sensitivity of the nuclear fission ionization box, the nuclear fission ionization is irradiated with neutrons by utilizing, for example, an atomic furnace, and the neutron sensitivity is obtained on the basis of the relationship between the amount of the irradiating neutrons and the current flowing between the anode and the cathode of the nuclear fission ionization box.
The conventional method gives rise to the problems that the number of atomic furnaces or the like used for irradiating the nuclear fission ionization box with neutrons is decreased, and that it is necessary to transfer the nuclear fuel substance to the outside of the radiation supervising area. In addition, the workers suffer from a radiation exposure accompanying the neutron irradiation, resulting in increases in the number of steps and cost for monitoring the neutron sensitivity.
A method that may overcome the above-noted problems is proposed in, for example, Japanese Patent Publication (Kokoku) No. 63-56956. It is proposed that a specified element is mixed the nuclear fission substance attached to at least one of the anode and the cathode. In this case, voltage is applied between the anode and the cathode so as to monitor the xcex1-ray current without actually irradiating the nuclear fission ionization box with neutrons, and the soundness of the nuclear fission type neutron detector and the neutron sensitivity are examined on the basis of the xcex1-ray current characteristics. In this method, however, it is difficult to obtain the neutron sensitivity with a high accuracy because of, for example, the nonuniformity in the size between the electrodes of the neutron detector and in the insulation resistance generated in the cable. Therefore, a further improvement is required.
An object of the present invention is to provide a method of monitoring the neutron sensitivity of a neutron detector, which permits monitoring with a high accuracy the neutron sensitivity of the neutron detector without involving the neutron irradiation.
According to one embodiment of the present invention, there is provided a method of monitoring the neutron sensitivity of a neutron detector, in which at least one of an anode and a cathode arranged to face each other is coated with a nuclear fission substance containing a parent nuclear species and an ionizing gas is loaded in the space between the anode and the cathode, comprising the steps of applying voltage of a predetermined potential difference between the anode and the cathode under the state that the nuclear detector is not irradiated with neutrons so as to measure an xcex1-ray current (Ixcex1) flowing between the anode and the cathode by the ionization of the ionizing gas caused by the xcex1-ray emitted from a nuclear fission substance; and obtaining, with the monitoring region in which the applied voltage and the xcex1-ray current (Ixcex1) bear a substantially proportional relationship within at least 50V of the applied voltage used as a reference, an extrapolated zero-volt xcex1-ray current (Ixcex10) at 0V of the applied voltage between the anode and the cathode from the proportional relationship by an extrapolating method, and by correlating the extrapolated zero-volt xcex1-ray current (Ixcex10) with the neutron sensitivity.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.